272 ROLLIN D. SALISBURY 



the changes which followed are less impressive than would have been 

 expected. The data at hand do point to extensive migrations, but 

 not to the exterminations and profound modifications which might 

 have been anticipated. It seems impossible to think that the changes 

 of climate which drove musk oxen to Kentucky and Virginia, and 

 Arctic plants and reindeer to the lowlands of central Europe and to the 

 Mediterranean, were without very profound biologic significance, 

 unless the life of the earth had reached a condition of far greater 

 stability than that of earlier times, when lesser physical changes seem 

 to have produced greater biological changes. 



One of the features of the late Tertiary land life, and especially 

 of the floras, seems to have been the great extent to which types were 

 mingled. This mingling of tropical or sub-tropical forms with 

 temperate and boreal ones seems to have begun as early as the middle 

 of the period. The oscillations of climate which marked the Pleisto- 

 cene seem to have had a sifting influence upon the migratory forms, 

 and to have forced them to special adaptations and habitats as the 

 period progressed. This is suggested, for example, by the floras of 

 America and Eurasia. Gray pointed out long ago that the forest 

 flora of the eastern part of North America is more like that of Japan 

 than like that of the western part of our own continent. In Europe, 

 the north-south and south-north migration of the floras as ice-sheets 

 advanced and receded was interfered with by the east-west mountain 

 ranges and by the seas bordering Europe on the south. In eastern 

 Asia and America, on the other hand, the back-and-forth migration 

 of the floras was facilitated by the greater continuity of land between 

 high and low latitudes, and in America at least, by the absence of 

 east-west mountain ranges. In the western part of the United States, 

 the irregular topography made repeated latitudinal migrations of the 

 floras more difficult than in the eastern part, though perhaps less 

 difficult than in Europe. In eastern Asia and in eastern America, 

 where migration was relatively easy, the forest flora is much larger 

 than in Europe or western North America. Thus Atlantic America 

 and Pacific Asia have each 66 genera of forest trees, while Pacific 

 America and Europe have but 31 and t,^ genera respectively, and the 

 number of species is approximately in keeping with the number of 

 genera. 



